The easiest way to think of a portion, as a general rule, is that it fits in the palm of your hand, i.e. chicken breast, etc. Vegetables vary depending on the vegetable.......most are 1/2 cup is a serving, peas 2/3 c is s serving (lots of space between the peas, I guess), look at LABELS. They will tell you how much a serving is....4 crackers, etc. You need to also read labels for CONTENT.......total calories, sodium, fat (SAT FAT is the most important thing to look at, not necessarily total fat), carbs, sugar, fiber and protein.
When you're looking at fat, you may see that it has 9 gms of fat and only 1 gram of sat fat. THAT is a good thing. As a rule, NOTHING should be more than 30% fat. In other words, if something has 210 calories, no more than 70 calories should come from fat. There are good and bad fats. It takes good fats to break down carbs AND are essential for the skin, organs, etc.
7 calories /gram of alcohol, 4 calories/gm of carbs, 4 calories / gm of protein and 9 calories per gm of fat. (THATS why you watch your fat intake. They have more than double the calories per gram of carbs and protein.
Make SURE to get plenty of protein. You won't be as hungry and it keeps your from craving sweets. If you crave sweets, you're not getting enough protein.
Eat at least 4-5 x during the day - mini meals/grazing. If you eat 250 calories 5 x a day, that's still only 1250 calories. Try to get protein and fruit or vegetable at EACH of the 4 or 5 times. For example, boiled egg and grapefruit for breakfast, apple and almonds for snack, 1/2 turkey sandwich and raw veggies for lunch, 1/2 large banana w/ almond or peanut butter for a snack, soup and salad for dinner, making sure you have kidney beans, black beans, nuts, peas, etc in the salad if the soup doesn't have any meat. Even if it does, it will just add more protein.
As far as the back pain goes.........go to the EGOSCUE website and pull off some e-cises for back pain. Getting the weight off will help, but you're probably out of alignment. you don't necessarily need to go to the chiro, just get the muscles re-educated to bring you back into proper alignment and out of pain. Might also want to get a copy of PAIN FREE by Pete Egoscue. VERY user friendly!!
hope this helps!